Proposals have already been made to manufacture objects based on silicon nitride by sintering a silicon nitride powder to which sintering aids such as yttrium or magnesium oxide have been added. However, silicon nitride powder is extremely expensive and is difficult to obtain in the pure state. To perform natural sintering without recourse to hot pressing, the starting powder must be very fine, which increases the risk of oxidation by the oxygen in the air and of dissolution in the water with which it comes into contact during processing. Once oxidised the powder becomes difficult, and in some cases impossible, to sinter.
Proposals have also been made in French published patent specification No. 2 353 503 to manufacture an oxynitride of silicon and aluminum of the so-called SiAlON type, by nitriding and sintering a mixture of aluminum powder, silicon powder and alumina powder to form an intermediate product, by crushing the powdered intermediate product with powdered silicon nitride and a small quantity of sintering additive such as magnesia, and finally by sintering the mixture to produce a dense ceramic product. After nitriding such a method gives an intermediate product which is very difficult to re-crush, and it does not provide a sintered material whose composition is close to that of silicon nitride, but rather one which is rich in aluminum and contains a notable quantity of oxygen.